For more than a century, biology textbooks have stated that vision among vertebrates - people included - is built from two clearly defined cell types: rods for processing dim light and cones for bright light and color. New research involving deep-sea fish shows this tidy division is, in reality, not so tidy.
Scientists have identified a new type of visual cell in deep-sea fish that blends the shape and form of rods with the molecular machinery and genes of cones. This hybrid type of cell, adapted for sight in gloomy light conditions, was found in larvae of three deep-sea fish species in the Red Sea, Reuters reported.
The species studied were: a hatchetfish, with the scientific name Maurolicus mucronatus; a lightfish, named Vinciguerria mabahiss; and a lanternfish, named Benthosema pterotum. The hatchetfish retained the hybrid cells throughout its life. The other two shifted to the usual rod-cone dichotomy in adulthood.
All three are small, with adults measuring roughly 1-3 inches (3-7 cm) long and the larvae much littler. They inhabit a marine realm of twilight conditions, with sunlight struggling to penetrate into the watery depths.
The vertebrate retina, a sensory membrane at the back of the eye that detects light and converts it into signals to the brain, possesses two main types of light-sensitive visual cells, called photoreceptors. They are named for their shape: rods and cones.
"The rods and cones slowly change position inside the retina when moving between dim and bright conditions, which is why our eyes take time to adjust when we flick on the light switch on our way to the restroom at night," said Lily Fogg, a postdoctoral researcher in marine biology at the University of Helsinki in Finland and lead author of the research published in the journal Science Advances.
"We found that, as larvae, these deep-sea fish mostly use a mix-and-match type of hybrid photoreceptor. These cells look like rods - long, cylindrical and optimized to catch as many light particles - photons - as possible. But they use the molecular machinery of cones, switching on genes usually found only in cones," Fogg said.
The researchers examined the retinas of fish larvae caught at depths from 65 to 650 feet (20 to 200 meters). In the type of dim environment they inhabit, rod and cone cells both are usually engaged in the vertebrate retina, but neither works very well. These fish display an evolutionary remedy.
"Our results challenge the longstanding idea that rods and cones are two fixed, clearly separated cell types. Instead, we show that photoreceptors can blend structural and molecular features in unexpected ways. This suggests that vertebrate visual systems are more flexible and evolutionarily adaptable than previously thought," Fogg said.
"It is a very cool finding that shows that biology does not fit neatly into boxes," said study senior author Fabio Cortesi, a marine biologist and neuroscientist at the University of Queensland in Australia. "I wouldn't be surprised if we find these cells are much more common across all vertebrates, including terrestrial species."
All three species emit bioluminescence using small light-emitting organs on their bodies, mostly located on the belly. They produce blue-green light that blends with the faint background light from the sun above. This strategy, called counterillumination, is a common form of camouflage in the deep sea to avoid predators.
"Small fish like these fuel the open ocean. They are plentiful and serve as food for many larger predatory fishes, including tuna and marlin, marine mammals such as dolphins and whales, and marine birds," Cortesi said.
These kinds of fish also engage in one of the biggest daily migrations in the animal kingdom. They swim near the surface at night to feed in plankton-rich waters, then return to the depths - 650 to 3,280 feet (200 to 1,000 meters) - during daytime to avoid predation.
"The deep sea remains a frontier for human exploration, a mystery box with the potential for significant discoveries," Cortesi said. "We should look after this habitat with the utmost care to make sure future generations can continue to marvel at its wonders."
